Boreholes are frequently drilled as vertical holes, and prior art provides sufficient technology to frequently achieve continued alignment along a single axis, both when the drilling string penetrates the ground and upon withdrawal.
However difficulties are encountered when boreholes are drilled firstly vertically downwardly, and then curve to an inclined or horizontal direction, and maintaining of complete control is very important but very difficult to achieve. A borehole drill may, for example, comprise three cutters which are circumferentially spaced from one another, and when the drill bit is inclined to the original axis, the space between the cutters can cause erratic or irregular operation in negotiating a curve. This is slightly improved by the use of rotary roller reamers, and the most relevant prior art to this invention known to the Applicant is disclosed in its own two previous inventions, respectively Australian Patent 594885 and Application PM 2305 (PCT/AU94/00691). The former Application disclosed a reamer having a plurality of hard inserts projecting therefrom (or continuous with the outer surface of the reamer in some instances), and the reamer was carried in respective end blocks in a recess in a reamer body. The arrangement for retention of the blocks utilising wedge surfaces was an important aspect of that invention. However even with the improvements which have been disclosed in the above specifications, difficulty is still encountered when the drill bit is required to traverse a curve, particularly upon withdrawal, when the string tends to straighten. The tendency to straighten sometimes causes the drill string to lock into the borehole, whereupon abandonment is unavoidable. This sometimes occurs even when a curve is not intentionally traversed. Since the cost of a drill string is very high, it is the main object of this invention to provide improvements which will make it easier to control a drill string when in a curved portion of a borehole.